Fossils are known from the Cynognathus Assemblage Zone of the Beaufort Group (part of the Karoo Supergroup) in South Africa and the Rouse Hill Siltstone of Australia that date back to the Anisian stage of the Middle Triassic.

[4] When found, the skull was described as an "embedded palate upwards, in a fairly soft dark-green shaly mudstone" which is characteristic of the amphibious behavior of Microposaurus.

During an expedition in the Anisian Rouse Hill Siltstones, Steven Avery found and A. Anne Warren named the holotype and single specimen of Microposaurus averyi.

[5] Besides being narrow, long, and wedge-shaped like most trematosaurids, M. casei also had significant ossification that caused individual skull bones to be indistinguishable from fusion with one another.

[1] The rounded tip of their short snout produced nostrils that were closely spaced and further back which was from a significant prenarial growth.

[1] Found in all trematosauroids, their orbits were elliptical (long axes oriented medially) and had a smooth dorsal surface to the palatines.

[1] The palatal tooth row were reduced with little teeth on the vomers (medial to the choana) and on the posterior ends of the ectopterygoids.

Common trematosaurines features observed from the specimen were their orbits were within the anterior half of their skull, the postorbital-prepineal growth zone was present, the anterior palatal vacuities were paired, the transvomerine tooth row was reduced or absent, the parasphenoid was antero-posteriorly elongated, the exoccipitals were underplated by the parasphenoid being posteriorly expanded, and in adults the orbits were small and located near the lateral margins of the skull.

The physically striking contrast between Microposaurus and Lonchorhynchinae was their seemingly short snouts compared to the elongated counterpart of the latter.

While Microposaurus lived during the Anisian period of the Triassic, the transition from the Permian-Triassic created more aquatic environments for these species to survive.

In the Karoo Basin of South Africa, after the extinction, multiple new rivers were created that ran through areas of the continent.

[7][2] The contribution to this wide distribution is from their possible euryhaline ability and preference to "nearshore marine to distal deltaic habitats".